If Apple allows in-app purchases freely, what may happen is that every app is a free app, and the first screen is a screen asking you to buy the paid app. I have already experience a similar experience in some apps like NYT where basically you need in-app subscription. As a user, I felt that I was faked into thinking that it is a free app. On the other hand, it is also crazy for Apple to prevent any sort of in-app upgrades.